I tried to close several completed issues by selecting them and using "WITH SELECTED ISSUES" action "Set Status" and set to "CLOSED". Their status now shows up as "CLOSED" but they are still in the "Open issues" queue and the name isn't in strikeout font like the other closed issues. So I conclude there is a open/closed flag somewhere but IO don't know how to set it other than by using the "Resolve issue" button in the issue edit page.

(There is a "Resolve issue" in the "Choose workflow step to perform..." action but it doesn't work: Just shows a box containing the words "RESOLVE THE ISSUE" and does nothing else, as if half the dialog (including a submit button) is missing.)